The present invention relates to a data conversion apparatus such as for use in coupling of a G3 type facsimile machine to a digital communication line such as Integrated Service Digital Network (ISDN), or for use in coupling of a G4 type facsimile machine (adopted to the digital communication line) to an analog communication line.
Recently, various new terminal devices such as the G4 type facsimile machine adopted for use in the digital communication line are developed in view of commercialization of a fast digital network. On the other hand, existing terminal devices such as the G3 type facsimile machine can be coupled to the digital communication line by means of a terminal adapter such that a digital data from the digital communication line is once converted into an analog signal, which is then fed to an existing interface such as V-series or X-series recommended by CCITT (Comite Consultatif International Telegraphique et Telephonique). Further, the existing terminal devices adopted for the analog communication line may undergo internal signal processing based on digital data, hence these terminal devices may admit a digital data from the fast digital communication line after converting a sampling frequency thereof for the analog terminal devices. Conventionally, such a data conversion is carried out by following methods:
(1) A sampling frequency of an input digital data is provisionally raised by oversampling, and thereafter interpolation is applied to the oversampled input digital data. PA1 (2) Interpolation of higher order is effected according to Lagrange's interpolation formula. PA1 (3) An input digital data is subjected to one-bit D/A conversion by sigma-delta modulation to produce a bit stream output, which is then decimated to thereby obtain a desired sampling frequency.
However, these conventional data conversion methods suffer from drawbacks such as a vast computation amount and a complicated circuit construction are required.